The present invention relates generally to can end production apparatus and, more particularly, to an apparatus for use in association with a can end press for detecting product defects associated with can end tab mounting.
Modern aluminum beverage cans consist of a can body and a sealingly attached can end. The can body comprises a bottom wall and an integrally formed cylindrical sidewall which terminates in a top circular opening. The can end has a generally circular shape and is sealed around the upper opening of the can body after filling of the can body with a beverage. Prior to mounting of a can end on a can body, a can end may undergo a series of processing steps for providing a pull ring-type opening system for the can. Such a pull ring-type opening system is commonly referred to as a "pop top" and includes a pull ring or "tab" which is riveted to the top of the can end. The tab is grasped and pulled away from the can to cause rupture and subsequent opening formation in a scored portion of the can end. To provide such a pull ring-type opening configuration, can end blanks are operated upon by a can end die press. A can end blank, prior to entering a die press, consists of a thin, flat, circular piece of sheet metal having no surface modifications other than a curved, generally upwardly projecting peripheral flange.
The can end passes through a series of different operating stations as it moves through the die press. Different metal forming operations are performed at the different stations. A can end die press is described in U.S. Pat. No. 4,608,843 of Grims for CONVERSION DIE WITH DOUBLE END SENSOR and in U.S. patent application Ser. No. 219,203 of Gold et al. filed on the same date as the present application, for MONITOR AND CONTROL ASSEMBLY FOR USE WITH A CAN END PRESS, both of which are hereby specifically incorporated by reference for all that is disclosed therein. Typical can end die press stations may include a first station for sensing the presence or absence of a can end in the press; a second "bubble-down" station where a downwardly projecting "bubble", which is subsequently formed into a rivet, is formed at the center of the can; a third "bubble-up" station where the downwardly projecting, rivet-forming bubble is inverted to become an upwardly projecting, rivet-forming bubble; a fourth "emboss station" where various printing or other indicia are stamped into the can end; a fifth "score station" where the metal is scored to define the area of the end which is to be ruptured during subsequent opening by a consumer; a sixth "deboss station" at which a downwardly or upwardly extending offset is stamped in the can end to take up metal slack in the can end, and a seventh staking station at which a tab is "staked" to the can end, i.e. attached to the upwardly extending, rivet-forming bubble.
The die press compr-ises a lower fixed die shoe and an upper reciprocating die shoe which moves vertically with respect to the lower fixed die shoe. Each operating station comprises a lower portion mounted on the lower fixed die shoe which receives and holds a single can end thereon during each press stroke. Each operating station also comprises an upper portion mounted on the upper reciprocating die shoe which contains tooling which strikes a can end mounted on the associated lower portion at the bottom of each press stroke. Can ends are progressively moved from station to station between press operating strokes by a transfer assembly. Modern end presses may have a plurality of identical press "lanes," each lane comprising an identical series of operating stations as described above. Typical press operating speeds in a modern can end plant are on the order of 300 strokes per minute.
During press operation tabs are fed into the die press staking station for staking to the can ends. Occasionally, a tab will not feed or will feed improperly, causing a can end to leave the press without a tab. A further problem associated with the absence of a tab on a can end in a misfeed operation is that the misfed tab may be jostled and bounced from station to station within the press, resulting in random scoring of a can end at the station in which the loose tab alights. This random scoring associated with a loose tab end bouncing through the die press may result in "leakers," i.e. ruptured can ends which cause beverage leakage in subsequently formed cans. A misfeed may also occur through presenting two tabs rather than a single tab at the staking station, resulting in the staking of two tabs rather than a single tab to the can end. Such double-staking often causes rupture of the metal of the associated can end, causing a can formed with the double-staked can end to leak.
A number of systems have been designed for use in association with a can end die press for detecting tab-staking malfunctions. One prior art apparatus which is provided at a separate die press station immediately following the tab-staking station consists of a spring-loaded plunger apparatus which extends downwardly from the upper portion of the die press a sufficient distance to make yielding contact with a can end mounted on the lower portion of the die press when the press is at the bottom of an operating stroke. An orifice is provided at the bottom of the plunger device, which is attached in fluid communication with a vacuum source. If a can end with which the plunger makes engagement has a tab properly mounted thereon, the plunger device engages the tab in a manner which provides a space between the plunger end and the flat surface of the can end which thus enables air flow into the orifice at the end of the plunger to remain substantially uninterrupted. However, if no tab is present and the plunger device engages the flat surface of the can end, air flow into the plunger orifice is interrupted and the resulting pressure change within the air flow system acts as a signal to indicate the presence of a can end without a tab. One problem with this system has been that the response time of the system: the time between plunger/can end engagement and error signal generation, is relatively slow. At the high operating speeds of modern presses, such a slow response time may make the system inoperative, i.e. the duration of any air flow interruption signal is so short as to be difficult to detect at high operating speeds. Another associated problem is that the sensitivity of the pressure system must be adjusted for various press operating speeds. If an operator sets the pressure too high, then a series of false product defect signals are generated. If the pressure is set too low, product defects are not detected at all. A further problem with this system is that it is unable to detect double-staked tabs.
Another prior art system also relies on a spring-loaded plunger provided at a separate operating station immediately downstream of the tab-staking operating station. In this system, a metal detecting device is mounted above the upper terminal metal end of the plunger device and detects the plunger device when it is caused to be deflected upwardly by striking of a tab mounted on the can end. If the plunger device is not deflected upwardly, then the metal proximity sensor is not actuated. Thus, in this system, the absence of a metal detection signal is treated as the detection of a missing tab. One problem with such a system is that metal proximity sensors are quite sensitive and are subject to malfunction when the surrounding electromagnetic field changes, such as may be caused by a press door being left open rather than closed, etc. Thus, the sensitivity of such a metal sensing unit as well as the physical position of the plunger must be carefully adjusted and the device is subject to malfunction when the these adjustments are incorrect. Another problem with such a device is that it is incapable of detecting a double tab staked to a can end, i.e. the double-staking of a can end causes the plunger to be deflected a relatively greater distance upwardly than is usual, but this additional deflection is not distinguished by the metal proximity sensor which merely senses the presence of metal, whether through the deflection associated with a single tab or through the deflection associated with a double tab.
Another device which has been used with some die presses to detect the absence of a tab on a can end includes a pair of electrical sensors, one positioned above the can end and one positioned below the can end, which measure the electrical density through the can end in the region of the tab. A problem with this system is that it cannot be used within the harsh operating environment of the die press apparatus itself, but must rather be used outside of the die press. Such a device may not be used in many existing press designs because of inherent problems associated with transferring can ends out of and then returning can ends to the ordinary production flow from the die press.
Thus, a need exists for a tab sensing device which may be positioned within a die press apparatus and which is capable of accurately determining whether a tab is properly mounted on a can end.